Geographical and temporal trends and seasonal relapse in Plasmodium ovale spp. and Plasmodium malariae infections imported to the UK between 1987 and 2015.

Background: Plasmodium ovale spp. and P. malariae cause illness in endemic regions and returning travellers. Far less is known about these species than P. falciparum and P. vivax.

Methods: The UK national surveillance data, collected 1987 to 2015, were collated with the International Passenger Survey and climatic data to determine geographical, temporal and seasonal trends of imported P. ovale spp. and P. malariae infection.

Results: Of 52,242 notified cases of malaria, 6.04% (3157) were caused by P. ovale spp. and 1.61% (841) by P. malariae; mortality was 0.03% (1) and 0.12% (1), respectively. Almost all travellers acquired infection in West or East Africa. Infection rate per travel episode fell fivefold during the study period. The median latency of P. malariae and P. ovale spp. was 18 and 76 days, respectively; delayed presentation occurred with both species. The latency of P. ovale spp. infection imported from West Africa was significantly shorter in those arriving in the UK during the West African peak malarial season compared to those arriving outside it (44 days vs 94 days, p < 0.0001), implying that relapse synchronises with the period of high malarial transmission. This trend was not seen in P. ovale spp. imported from East Africa nor in P. malariae.

Conclusion: In West Africa, where malaria transmission is highly seasonal, P. ovale spp. may have evolved to relapse during the malarial high transmission season. This has public health implications. Deaths are very rare, supporting current guidelines emphasising outpatient treatment. However, late presentations do occur.